Apple Compressor 3 - Manual (Page 147)

The file is now assigned to the “L” (Left Front) Channel.
3 Repeat step 2 for each of the source audio files that you intend to include in the surround
stream.
Note: If you are creating Dolby Digital Professional (AC-3) surround sound streams, you
will not use all the channels listed in the table at once. See
Audio Tab Settings for a
diagram of the Dolby audio coding modes.
4 When you have finished adding source audio files to the channel assignment interface,
click OK.
147 Chapter 10 Creating Dolby Digital Professional Output Files